For oilfield and hydrocarbon exploration it is particularly useful to have a tool that is capable of scanning a subsurface geological formation and to convey data representing the various strata and hydrocarbons that constitute a subsurface geological formation. Specifically, after drilling a borehole down into the earths crust, it would be useful to have downhole tools that are capable of being run along the borehole wall and scanning the surrounding formation to provide an image of the formation's properties to a user on the surface. Equally, it is useful to have such a tool mounting on or close to a drill tip so that the formation can be imaged as the drill penetrates into the earths crust. This would enable a user to measure and/or image various formation parameters close to or ahead of the drill bit and from there get the latest information about the downhole formation, which might impact on the direction being drilled.
Tools using current injection are known, for example U.S. Pat. Nos. 4,468,623, 4,614,250, 4,567,759, 6,600,321, 6,714,014 or 6,809,521; that use current injection measurements in order to obtain micro-electric images of a borehole wall, the borehole penetrating geological formations.
Such tools inject AC current into the formation from one or more small electrodes (called “buttons”) and measure the current from each button and the voltage between the imaging buttons and the return electrode. In conductive mud (for example, water-based) the imaging button is surrounded by a guard electrode to force the current into the formation. In non-conductive (oil-based) mud such a guard is not necessary if the formation is more conductive than the mud at the frequency of operation. The imaging buttons plus guard electrode (if present) compose the injector. The impedance (voltage/current) seen by each button is indicative of the resistivity of a small volume of formation in front of each button. The area of the return electrode is usually much larger than the size of the injector, in order that the current tube spreads out between the injector and return to ensure first a high sensitivity and good resolution in front of the imaging buttons and second low sensitivity and resolution in front of the return electrode.
Such tools can be adapted for wireline use, in which an array of imaging buttons is at equipotential with a guard electrode on a pad (laterolog principle) and the return is on a distant part of the tool mandrel. Such tools operate at frequencies in the range 1-100 kHz where the formation generally has a resistive character and dielectric effects can be neglected except at very high resistivities.
Such tools can be adapted as logging-while drilling tools, which are able to achieve full coverage of the borehole with a limited number of electrodes by drill-string rotation. Laterolog principles are used, sometimes with additional focusing by hardware or software.
However, the use of such tools in non-conductive oil-based mud is of limited use because the impedance measured is generally dominated by the mud impedance between the injection electrode and formation that is in series with the formation impedance. Reasonable images can be obtained in high-resistivity formations, i.e. above about 1000Ω·m, but poor images result in formations having a lower resistivity.
Broadly speaking, two approaches have been adopted to enable better imaging through oil-based mud in formations of low resistivity.
The first approach relies on a different measurement principle, the four-terminal method as described in U.S. Pat. No. 6,191,588. Here the current is generated in the formation using two large electrodes near the ends of a pad and potential differences in the formation are measured using pairs of small electrodes at the centre of the pad. Using this technique the resolution is worse than conventional current injection tools because it is determined by the separation of the pair of voltage electrodes (rather than the size of the current injection electrode). Also, this technique is insensitive to events (bedding, fractures etc) parallel to the current flow (usually parallel to the borehole axis).
The second approach is to increase the frequency of injection-type tools in order to reduce the mud impedance, i.e. U.S. Pat. No. 2,749,503.
At high frequencies, various processing techniques have been suggested to reduce the influence of the non-conductive mud between the pad and the borehole. U.S. Pat. No. 7,066,282 proposes measuring the real part of the impedance seen by the button, while U.S Pat. Nos. 6,809,521, 7,394,258 and 7,397,250 all require making at least one mathematical approximation based on the mud impedance being essentially imaginary, or the formation impedance being essentially real, or using more than one frequency and assuming various mud properties are independent of frequency. These approximations have limited ranges of validity, since they do not adequately account for the electrical properties of the rocks and muds.
It is therefore desirable to provide a tool that is able to reduce the influence of the non-conductive mud medium when using a current injection principle and to avoid the previously-mentioned limitations.